It drives me nuts when people consistently make statements or in some other way incorporate faulty beliefs or incorrect facts that I know 100% for certain aren't right. I don't like being wrong, so other people should be equally adverse to it! This is just one of many things that drives me nuts because I know it's wrong--and why.
When movies, TV shows, books, or anything else use the plot device of longtime childhood friends who realize their attraction and fall in love and get married and live happily ever after. I hate it when people do this because, for all its more or less constant use in the media, this doesn't actually happen in real life. This isn't a case of me going, 'Oh, well, I don't like this so no one else should!' but a legitimate case of there being actual scientific theories (which are, by the way, incredibly fucking sound and completely correct, just like the theory of gravity and the theory of evolution) and evidence to back it up.
And we can all thank the nineteenth-century researcher Edvard Westermarck for working it out.
His theory, first published in 1891, was named the Westermarck Effect for him and simply states that you are programmed to be repulsed by anyone you grow up alongside. Westermarck developed his hypothesis by studying Israeli tribes who raised their children communally in groups according to age--kind of like grade school. He found that marriages were almost invariably between people who did not belong to the same age group--subsequent research has found this to be true across the board. Everywhere. It doesn't matter where you come from, your culture, or your sexual preferences. You are more or less guaranteed not to be in any way sexually or romantically interested in people you grew up with. Of course there are exceptions, but these are, well, exceptional--few, far between, and abnormal. Think about the kids you went to school with, especially if you lived in a very small community where you had the same classmates your entire school career. How often did students from the same grade become and remain romantically involved? I remember very few; when it happened, it was between students who were new or students who simply hadn't known each other long. And that's all Westermarck's Effect manifesting.
Nobody knows exactly why this happens but the general consensus is that it's some sort of psycho-evolutionary safety net designed to discourage us from inbreeding, which prevents us from perpetuating bad genes and destroying our genetic variability and doing inconvenient things like going extinct. Through as an aside, as genetic variability goes humans are pretty freaking similar. 99.9% of all our DNA is identical--the .1% that varies is what's used in genetic testing. Whether you like it or not, you are pretty goddamn closely related to everyone on the face of the planet. You have more genetically in common with the tribes of the Amazon Basin or anyone else (including, it's worth pointing out, your romantic and sexual partners, all of them) than, for example, any two chimpanzees who live in the same troupe. Again, nobody is quite sure why this is. The most likely answer is that at some point in our semi-recent (within the last few hundred thousand to a million years) history there was some kind of mass extinction event that wiped out all but a few of the humans on earth. It was either pure dumb luck or these lucky survivors had some unknown and unidentified genetic trait that offered an advantage against--or protection from--whatever killed everyone else. Either way, they were all probably closely related, meaning that only their very close DNA survived.
But still, that .1% of our DNA is what stops us from dying out and evolution is quite clever and has a lot of safety nets in place designed to stop you from committing incest and creating creepy mutant offspring with webbed feet. Think about your opposite-sex siblings, if you have them--or someone you know who has opposite-sex siblings. Especially if you're a girl bitching about your brothers. What's a common complaint? That their brothers smell disgusting. But unrelated people don't notice it. Why? Because you're actually having a negative hormonal reaction to your siblings. Your body is programmed to be disgusted by the scent of your siblings so you're less likely to have sex with them.
The thing about the Westermarck Effect is that it isn't tied to your biological siblings, as Westermarck's research in the Israeli tribes confirmed. It just so happens that the children you are most likely to be the closest to in your youth are also most likely going to be close relatives--siblings, half-siblings, cousins, and others with whom it would be reproductively dicey to have children. But being a psychological--rather than physical--effect, it makes no distinction between adopted siblings and blood relatives. If you grow up with them, you find them sexually repulsive.
Even the places where the Westermarck Effect seems to have totally failed to take hold, it still often does a good job as evidence that the theory is correct. Blood siblings who have never met--adopted or raised separately--who meet later in life sometimes develop a sexual attraction to one another. It's because, having never known one another in childhood, the Westermarck Effect never took hold.
It doesn't last forever. It appears that, on average, the Westermarck Effect dissipates by the time you're six or so, so anyone who came into your life after that is less likely to seem so romantically off-putting. It seems kind of young--after all, siblings and cousins can be much further apart than that in age. Just one of mine is less than seven years younger than me. But the point still stands: if you grow up around someone, you think of them as siblings, and as such you unconsciously reject them as sexual partners because your brain is trying to make sure you don't do something stupid, like boink your cousins.
Romances between childhood friends can and do happen. Just very infrequently, and such pairings don't often last long. It's just one of many things most people don't realize their bodies and minds are doing in order to make sure they do what's evolutionarily best for the species: be attracted to the right people, have sex with them often, and keep the gene pool from growing algae.
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